tubes are low-pressure Mercury vapor discharge lamps
which produce UV light.
The UV light hits the wall of a glass tube which is covered with Phosphor.
The Phosphor absorbs the UV light and transforms it to visible white light.
The lamp needs high voltage to start the discharge, and a device that limits the current during operation. Most FL lamps have therefore a ballast and a starter, in newer lamps these functions can be performed by electronic circuits (more expensive).